Natural microtubule encapsulated phase-change materials and preparation thereof

ABSTRACT

Microtubule encapsulated microcapsules of a phase-change material and preparation thereof are provided. The microcapsules of a phase-change material consist of a phase-change material, truncated microtubules, and a polymer. The truncated microtubules are formed by truncating hollow tubular natural fibers into fiber segments with a length of 0.1 mm-5 cm. The diameter of the hollow tubular natural fiber is 0.1-1000 μm. The phase-change material is encapsulated in the truncated microtubules and the truncated microtubules are covered with the polymer. The microtubules have high energy storage density due to high hollowness, and can transfer energy stably due to the closed structure, transfer heat rapidly due to the very fine micro-tubular structures, and may be used for a long term in view of the heat and chemical stability.

FIELD OF THE INVENTION

The present invention relates to natural microtubule encapsulated microcapsules of a phase-change material and the preparation thereof.

DESCRIPTION OF THE PRIOR ART

Generally, phase-change materials (PCM), also called as latent thermal energy storage (LTES) materials, refer to materials that are capable of absorb or release energy upon phase change while the temperature of the material does not change or change a little. When serving as an energy storage carrier, the phase-change materials have the advantages of high thermal storage density, small equipment volume, and high thermal efficiency, and heat absorption or release is a constant temperature process, thus the energy utilization can be improved, and the problem of energy crisis can be solved to some extent. Presently, phase-change materials have been widely used in refrigeration and cool storage of refrigerators and air-conditioners, automatic thermostatic control of smart buildings, energy storage and exchange technology in solar energy application, peak load shifting in power supply, recovery and reuse of waste heat and residual heat, and commodities. Due to simple and convenient use without energy consumption, the phase-change materials have wide application prospect and broad market.

In view of the phase change process of the material, the phase-change materials are mainly divided into solid-liquid phase-change material, solid-solid phase-change material, solid-gas phase-change material, and liquid-gas phase-change material. A large amount of gas exists during the phase change process of the later two materials, so that the volume change of the material is great, thus the two materials are rarely used in practical application. Due to small volume change, high latent heat, good energy storage, and wide phase-change temperature range, the solid-liquid phase-change material has been widely used in practice. However, as liquid phase is generated during the phase-change process, the material must be packed in a sealed container, so as to prevent leakage and environment pollution, and the container must be inert to the phase-change material. This disadvantage greatly limits the application of the solid-liquid phase-change energy storage materials in practice. Recently, with the development of technology and the requirement of application, people try to perform shape stabilization to convert the solid-liquid phase-change energy storage materials into solid-solid phase-change materials in form. However, solid-liquid phase change still occurs in practice, which solves the melting problem of the phase-change materials, and greatly facilitates the practical application. Presently, the method for performing shape stabilization process on the phase-change materials mainly includes shaped and microcapsulation.

The shaped phase-change materials are substantially composite phase-change energy storage materials, and refer to phase-change material having non-fluidity and capable of maintaining solid form formed by combining the phase-change material and the carrier material, which can substitute solid-solid phase-change materials. The phase-change materials contains two main components: one is working material component, that is, phase-change material, for storing and releasing energy through phase change, including various phase-change materials, with solid-liquid phase-change materials being mostly used; and the other is carrier material component, for maintaining the non-fluidity and processability of the phase-change material. Therefore, the melting temperature of the carrier material is required to be higher than the phase-change temperature of the phase-change material, such that the working material can maintain the solid shape and material performance in the phase change range. From the development of the compounding of the shaped composite phase-change materials in recent years, the main preparation method substantially includes: co-blending, grafting, sintering, in-situ intercalation, and sol-gel method. As the physical effect of the shaped phase-change material is relatively small, after being used repeatedly, the solid-liquid phase-change material may be easily desorbed from the carrier, and leakage and exudation, and two-phase separation may occur.

The microcapsulated phase-change materials are composite phase-change materials having a core-shell structure formed by covering the surface of the solid-liquid phase-change material particles with a layer of polymer membrane or inorganic material with stable performance by using microcapsule technology. During the phase change process, solid-liquid phase change occurs in the core of the microcapsulated phase-change material, while the outer layer polymer membrane maintains solid form, so the phase-change materials present as solid particles at macroscopic. The chemical preparation method of the microcapsules of a phase-change material mainly includes: in-situ polymerization, interfacial polymerization, reaction phase separation, and complex coacervation, and the shell performance obtained by different preparation methods is different. With the development of the polymer science, microcapsulation technology gets mature gradually, thus the phase-change energy storage microcapsule materials are widely concerned and researched due to the special performance and usage. The energy storage principle of heat absorption and release of the phase-change microcapsule is equivalent to that of a thermal battery. The encapsulation by the micro container makes the phase-change material converted into numerous small working units, thus significantly expanding the application field and situation of the phase-change material. The product with phase-change microcapsule blended therein will establish a microclimate environment in the melting point range of the phase-change material used, so as to meet the requirement for comfort on temperature. The microcapsulated phase-change materials can well solve the serious problems of easily melting and flowing, penetration and migration, phase separation, and corrosion during the solid-liquid phase change process, and after being encapsulated and protected with the shell material, the phase-change material is separated from the external environment to be stabilized. Also, the polymer shell material or the modified shell material significantly increases the compatibility of the phase-change material and the matrix material, thus significantly improving the practicality of the phase-change material. However, the strength of the microcapsule wall is insufficient, the leakage and heat resistance of the phase-change material still need to be improved, and particularly, the cost is high, which are the problems in urgent need to be solved in the industry presently.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a phase-change microcapsule of a truncated natural microtubule encapsulated phase-change material and the preparation thereof.

The microcapsules of a phase-change material of the present invention comprise a phase-change material, truncated microtubules, and a polymer. The truncated microtubules are formed by truncating hollow tubular natural fibers into fiber segments with a length of 0.1 mm to 5 cm. The hollow tubular natural fibers have a diameter in the range from 0.1 μm to 1000 μm. The phase-change material is encapsulated in the truncated microtubules, and the truncated microtubules are then encapsulated by the polymer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a DSC curve diagram of microcapsules of a phase-change material according to Example 1 under cyclic temperature rise and drop.

FIG. 2 is a scanning electron microscope photo of the microcapsules of a phase-change material according to Example 1, in which (A) is truncated natural kapok tubule; (B) and (C) are encapsulated kapok microtubules filled with paraffin; (D) is encapsulated paraffin-filled kapok tubule further encapsulated with urea-formaldehyde resin.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, useful natural fiber can be selected from kapok fiber, milkweed fiber, luffa fiber, bamboo fiber, tex bamboo fiber, flax fiber, wool, and down.

The phase-change material may be at least one of 1) solid-liquid phase-change material and 2) solid-solid phase-change material.

The solid-liquid phase-change material may be at least one of a) an inorganic phase-change material and b) an organic phase-change material.

According to the present invention, the inorganic phase-change material can be a crystalline hydrated salt and/or molten salt. The crystalline hydrated salt may be any one of alkaline metal or alkaline-earth metal halides, sulfates, phosphates, nitrates, acetates, or carbonates, or any combination thereof, such as, Na₂SO₄.10H₂O, Na₂HPO₄.12H₂O, CaCl₂.6H₂O, and SnCl.6H₂O and a combination thereof. The molten salt can be K₂WO₄ and/or K₂MoO₄.

The organic phase-change material can be any one of the following materials: higher aliphatic hydrocarbons, higher fatty acids, higher fatty acid esters, salts of higher fatty acids or esters, higher aliphatic alcohols, aromatic hydrocarbons, aromatic ketones, aromatic amides, fluorochloroalkanes, multicarbonyl carbonic acids, and crystalline polymers.

The higher aliphatic hydrocarbons are generally aliphatic hydrocarbons having 6 or more carbon atoms, preferably 6-36 carbon atoms. The higher fatty acids generally refer to C6-C26 mono-carboxylic acids.

The higher aliphatic hydrocarbon can be any of the following 16 substances or a combination thereof: n-octacosane, n-heptacosane, n-hexacosane, n-pentacosane, n-tetracosane, n-tricosane, n-docosane, n-henicosane, n-icosane, n-nonadecane, n-octadecane, n-heptadecane, n-hexadecane, n-pentadecane, n-tetradecane, and n-tridecane.

The crystalline polymer is high density polyethylene, polyvinylidene, or crystalline polyvinyl chloride having a density higher than 0.94 g/cm³.

The solid-solid phase-change material is an inorganic salt, a polyol, or a cross-linked polymer resin. The inorganic salt may be Li₂SO₄ and/or KHF₂. The polyol may be any of the following 6 substances or a combination thereof: pentaerythritol (PE), 2,2-bis(hydroxymethyl) propanol, neopentyl glycol (NPG), 2-amino-2-methyl-1,3-propanediol, trimethylolethane, and tris(hydroxymethyl)aminomethane. The cross-linked polymer resin may be a cross-linked polyolefin, a cross-linked polyacetal, a co-polymer of a cross-linked polyolefin and cross-linked polyacetal, or a blend of a cross-linked polyolefin and cross-linked polyacetal.

The polymer in the polymer layer of the microcapsules of phase-change a material of the present invention is any of the following 10 polymers or copolymers or blends thereof: urea-formaldehyde resin, melamine-formaldehyde resin, melamine-urea-formaldehyde resin, polyurethane, polymethylmethacrylate, poly(ethyl methacrylate), phenolic resin, epoxy resin, polyacrylonitrile, cellulose acetate.

According to the present invention, the truncated microtubule encapsulated phase-change microcapsules can be prepared by the method comprising the following steps:

1) Liquefying the Phase-Change Material:

-   -   heating the phase-change material to above the melting point or         dissolving it with a solvent, so as to obtain a liquid         phase-change material;         2) Filling Truncated Natural Microtubules with the Liquid         Phase-Change Material:     -   dispersing and immersing truncated natural microfibers into the         liquid phase-change material obtained in 1), so as to make the         microtubules filled with the liquid phase-change material         through capillary absorption; and

3) Encapsulating the Phase-Change Material:

-   -   encapsulating the microtubules filled with the phase-change         material obtained in Step 2) with a polymer, so as to obtain the         microcapsules of the phase-change material.

Optionally, the method further comprises washing off the phase-change material adsorbed on the surface of the resultant microcapsules of the phase-change material.

The solvent may be any one of the following 10 solvents or a mixture thereof: deionized water, N,N′-dimethylformamide, N,N′-dimethylacetamide, tetrahydrofurane, methylene chloride, trichloromethane, cyclohexane, methanol, ethanol, and acetone.

Compared with the existing microcapsulated phase-change material encapsulation technology, the present invention has the following advantageous effects:

1. The encapsulation tubules used in the present invention are cheap and easily available natural microfibers. For example, kapok fiber is a natural fiber having a large specific surface area and a high hollowness up to 80-90%, which is difficult to be realized by current artificial preparation methods, thus being more suitable for manufacturing phase-change energy material than man-made fibers. Further, kapok fiber has a high thermostability and substantially will not be thermally degraded at 250° C. Also, kapok fiber has a high chemical stability, and will only be dissolved in high-concentration strong acids.

2. The truncated natural microfibers having micropore structure with large specific surface area are used as supporting materials, and through the capillary force of the micropores, the liquid organic phase-change energy storage material or the inorganic phase-change energy storage material (at a temperature higher than the phase change temperature) is absorbed into the micropores, so as to form an organic phase-change energy storage material, inorganic phase-change energy storage material, or a composite of an organic and inorganic phase-change energy storage materials. When a solid-liquid phase change of the phase-change energy storage material occurs in the micropores, due to the capillary force, the liquid phase-change energy storage material will not easily overflow from the micropores.

3. Although the capillary force solves the fluidity problem of the solid-liquid phase-change material to some extent, it is still an “open” package system. Thus the microcapsulated microtubules with the phase-change material adsorbed therein can be further closed and terminated with a polymer.

4. The microfibers have a high hollowness and therefore a high energy storage density, and can transfer energy stably due to the closed structure, and transfer heat rapidly due to the very fine micro-tubular structures, and may be used for a long term in view of the heat and chemical stability. Further, the special lipophilic and hydrophobic wetting performance can be utilized during the processing.

5. The microcapsulated form of the phase-change material can be better dispersed in a matrix material during practical technical process. After being mixed with the matrix material, the micron-level size of the encapsulated phase-change material can make the appearance of the product be maintained and not be affected.

EXAMPLES Example 1 Preparation of Natural Kapok Fiber Encapsulated Paraffin and Urea-Formaldehyde Resin Encapsulated Microcapsules of a Phase-Change Material

(1) Liquefaction of Phase-Change Material:

The organic phase-change material paraffin was heated to above the melting point of 60° C. to obtain a liquid paraffin phase-change material.

(2) Filling Truncated Natural Microtubules with the Liquid Phase-Change Material:

1 g natural kapok fiber (truncated microtubules) having a length of 10-50 μm was dispersed into 10 mL liquid phase-change material obtained in Step (1), and immersed to make the capillary absorption reach a balance, such that the kapok fiber was fully filled with the liquid phase-change material.

(3) Encapsulation of Microcapsulated Phase-Change Material:

2 g urea-formaldehyde prepolymer (obtained by adding 1 g urea into 2 ml 36% volume fraction aqueous formaldehyde solution and stirring until the mixture was fully dissolved, heating to 60° C., and maintaining at this temperature for 15 min) was directly added dropwise into the melt of the phase-change material filled natural kapok fiber obtained in Step (2), the temperature of the melt was raised to 97-98° C. and the reaction lasted for 1 h. Urea-formaldehyde resin polymer was generated around the kapok fiber, and thus phase separation and deposition occurred, such that the microcapsulated phase-change material was encapsulated by the urea-formaldehyde resin.

(4) Purification of the Microcapsulated Phase-Change Material:

The urea-formaldehyde resin encapsulated and phase-change material fully filled microtubules obtained in Step (3) were taken out, and placed in hot water to wash off the phase-change material adsorbed on the surfaces of the tubules, and dried, so as to form the microcapsulated phase-change material.

The DSC curve of the phase-change material under cyclic temperature rise and drop is as shown in FIG. 1, and the scanning electron microscope photo of the phase-change material is shown in FIG. 2.

It can be seen from FIG. 1 that the microcapsules of a phase-change material have a good cyclic phase-change energy storage effect under cyclic temperature rise and drop.

It can be seen from FIG. 2 that the encapsulated kapok microtubules filled with paraffin form encapsulated phase-change materials after being encapsulated with the urea-formaldehyde resin.

Example 2 Preparation of Natural Milkweed Fiber Encapsulated Pentaerythritol and Cellulose Acetate Encapsulated Microcapsules of a Phase-Change Material

(1) Liquefaction of Phase-Change Material:

Organic phase-change material pentaerythritol (PE) was dissolved in a small amount of ethanol, to obtain a liquid pentaerythritol (PE) solution phase-change material.

(2) Filling Truncated Natural Microtubules with the Liquid Phase-Change Material:

1 g natural milkweed fiber having a length of 0.5-10 μm was dispersed into 10 mL liquid phase-change material obtained in Step (1), and immersed to make the capillary absorption reach a balance, such that the milkweed fiber was fully filled with the liquid phase-change material.

(3) Encapsulation of Microcapsulated Phase-Change Material:

The ethanol in the phase-change material microcapsule containing ethanol solvent obtained in Step (2) was vaporized, the phase-change material pentaerythritol (PE) solution was concentrated and solidified, and then immersed in 5 mL of 5 wt % cellulose acetate solution in methylene chloride, such that the microcapsulated phase-change material was encapsulated by cellulose acetate through interfacial deposition.

(4) Purification of Microcapsulated Phase-Change Material:

The cellulose acetate encapsulated and phase-change material filled milkweed fiber obtained in Step (3) was taken out and dried, so as to form the microcapsulated phase-change material.

The microcapsules of a phase-change material prepared according to this method have a good cyclic phase-change energy storage effect under cyclic temperature rise and drop, and the encapsulated milkweed microfibers filled with pentaerythritol (PE) form the encapsulated phase-change material with good dispersion after being encapsulated by cellulose acetate.

Example 3 Preparation of Natural Bamboo Fiber Encapsulated CaCl₂.6H₂O and Cellulose Acetate Encapsulated Microcapsules of Phase-Change a Material

(1) Liquefaction of Phase-Change Material:

1 g inorganic phase-change material CaCl₂.6H₂O was dissolved in 10 mL deionized water, to obtain a liquid CaCl₂.6H₂O solution phase-change material.

(2) Filling Truncated Natural Microtubules with the Liquid Phase-Change Material:

1 g natural bamboo fiber having a length of 500-1000 μm was dispersed in 10 mL liquid phase-change material obtained in Step (1), and immersed to make the capillary absorption reach a balance, such that the bamboo fiber was fully filled with the liquid phase-change material.

(3) Encapsulation of Microcapsulated Phase-Change Material:

The deionized water in the phase-change material microcapsules containing deionized water obtained in Step (2) was vaporized, the phase-change material CaCl₂.6H₂O solution was concentrated and solidified, and then immersed in 10 mL of 5 wt % cellulose acetate solution in methylene chloride, such that the microcapsulated phase-change material was encapsulated by cellulose acetate through interfacial deposition.

(4) Purification of Microcapsulated Phase-Change Material:

The cellulose acetate encapsulated and phase-change material filled bamboo fiber obtained in Step (3) was taken out and dried, so as to form the microcapsulated phase-change material.

The microcapsules of a phase-change material prepared according to this method have a good cyclic phase-change energy storage effect under cyclic temperature rise and drop, and the encapsulated bamboo microfibers filled with inorganic phase-change material CaCl₂.6H₂O form the encapsulated phase-change material with good dispersion after being encapsulated by cellulose acetate.

Example 4 Preparation of Natural Flax Fiber Encapsulated Pentaerythritol and Li₂SO₄ and Polyacrylonitrile Encapsulated Microcapsules of a Phase-Change Material

(1) Liquefaction of Phase-Change Material:

10 g organic phase-change material pentaerythritol (PE) and 10 g inorganic phase-change material Li₂SO₄ were dissolved in 10 mL mixed solution of deionized water and alcohol (50:50 v/v), to get a liquid organic/inorganic mixed phase-change material.

(2) Filling Truncated Natural Microtubules with the Liquid Phase-Change Material:

5 g natural flax fiber having a length of 100-500 μm was dispersed in 10 mL liquid phase-change material obtained in Step (1), and immersed to make the capillary absorption reach a balance, such that the flax fiber was fully filled with the liquid phase-change material.

(3) Encapsulation of Microcapsulated Phase-Change Material:

The deionized water and alcohol in the phase-change material microcapsule containing deionized water and alcohol solvent obtained in Step (2) were vaporized, the mixed pentaerythritol (PE) and Li₂SO₄ solution phase-change material was concentrated and solidified, and immersed into 5 mL of 5 wt % polyacrylonitrile solution in N,N′-dimethylformamide, such that the microcapsulated phase-change material was encapsulated by polyacrylonitrile through interfacial deposition.

(4) Purification of Microcapsulated Phase-Change Material:

The polyacrylonitrile encapsulated and phase-change material filled flax fiber obtained in Step (3) was taken out and immersed in deionized water to solidify the polyacrylonitrile, and then dried.

The microcapsules of a phase-change material prepared according to this method have a good cyclic phase-change energy storage effect under cyclic temperature rise and drop, and the encapsulated natural flax microfibers filled with the phase-change material pentaerythritol (PE) and Li₂SO₄ form the encapsulated phase-change material with good dispersion after being encapsulated by polyacrylonitrile. 

1. Microcapsules of a phase-change material, comprising: a phase-change material, truncated microtubules, and a polymer; wherein the truncated microtubules are formed by truncating hollow tubular natural fibers into fiber segments having a length of 0.1 mm-5 cm, and the hollow tubular natural fibers have a diameter of 0.1-1000 μm; the phase-change material is encapsulated in the truncated microtubules, and the truncated microtubules are encapsulated by the polymer.
 2. The microcapsules of a phase-change material according to claim 1, wherein the natural fiber is at least one of the following natural fibers: kapok fiber, milkweed fiber, luffa fiber, bamboo fiber, tex bamboo fiber, flax fiber, wool, and down.
 3. The microcapsules of a phase-change material according to claim 1, wherein the polymer is any one of the following polymers or copolymers or blends thereof: urea-formaldehyde resin, melamine-formaldehyde resin, melamine-urea-formaldehyde resin, polyurethane, polymethylmethacrylate, poly(ethyl methacrylate), phenolic resin, epoxy resin, polyacrylonitrile, and cellulose acetate.
 4. The microcapsules of a phase-change material according to claim 1, wherein the phase-change material is at least one of 1) a solid-liquid phase-change material and 2) a solid-solid phase-change material; the solid-liquid phase-change material is at least one of a) an inorganic phase-change material and b) an organic phase-change material; the inorganic phase-change material is a crystalline hydrated salt and/or molten salt; the organic phase-change material is any one of the following materials: higher aliphatic hydrocarbons, higher fatty acids, higher fatty acid esters, salts of higher fatty acids, higher aliphatic alcohols, aromatic hydrocarbons, aromatic ketones, aromatic amides, fluorochloroalkanes, multicarbonyl carbonic acids, and crystalline polymers; and the solid-solid phase-change material is an inorganic salt, a polyol, or a cross-linked polymer resin.
 5. The microcapsules of a phase-change material according to claim 4, wherein the crystalline hydrated salt is selected from: alkali metal halides, alkaline-earth metal halides, sulfates, phosphates, nitrates, acetates, carbonates, and combinations thereof; the molten salt is K₂WO₄ and/or K₂MoO₄; the inorganic salt is Li₂SO₄ and/or KHF₂; the higher aliphatic hydrocarbon is selected from: n-octacosane, n-heptacosane, n-hexacosane, n-pentacosane, n-tetracosane, n-tricosane, n-docosane, n-henicosane, n-icosane, n-nonadecane, n-octadecane, n-heptadecane, n-hexadecane, n-pentadecane, n-tetradecane, n-tridecane, and combinations thereof; the crystalline polymer is high density polyethylene, polyvinylidene, or crystalline polyvinyl chloride having a density of higher than 0.94 g/cm³; the polyol is selected from: pentaerythritol, 2,2-bis(hydroxymethyl)propanol, neopentyl glycol, 2-amino-2-methyl-1,3-propanediol, trimethylolethane, and tris(hydroxymethyl)aminomethane; the cross-linked polymer resin is a cross-linked polyolefin, a cross-linked polyacetal, a copolymer of a cross-linked polyolefin and cross-linked polyacetal, a blend of a cross-linked polyolefin and cross-linked polyacetal, and combinations thereof.
 6. A method for preparing the microcapsules of a phase-change material according to one of claims 1 to 5, comprising: 1) heating a phase-change material to above the melting point, or dissolving it with a solvent, so as to obtain a liquid phase-change material; 2) dispersing and immersing truncated microtubules into the liquid phase-change material obtained in Step 1), so as to make the microfibers filled with the liquid phase-change material through capillary absorption; and 3) encapsulating the truncated microtubules filled with the phase-change material obtained in Step 2) with a polymer, so as to obtain the microcapsules of the phase-change material.
 7. The method according to claim 6, further comprising washing off the phase-change material adsorbed on the surface of the obtained microcapsules of the phase-change material.
 8. The method according to claim 6, wherein the solvent is selected from: deionized water, N,N′-dimethylformamide, N,N′-dimethylacetamide, tetrahydrofurane, methylene chloride, trichloromethane, cyclohexane, methanol, ethanol, acetone, and mixtures thereof. 